Single photon avalanche diodes (SPADs) are used in a variety of applications, such as time of flight ranging systems, to detect incoming photons. When impinged upon by an incoming photon, a SPAD in combination with a detection circuit generates an output electrical pulse.
However, these output pulses are relatively “long”, lasting on the order of 10 ns or more. Due to these relatively long pulses, depending on the frequency of arrival of incoming photons, it is possible for the distinct output pulses of the SPAD to “pile up”. That is, due to the pulse lengths, the time between impinging of individual photons may be less than the length of the pulses, and thus individual events are no longer seen as the pulses overlap one another.
This results in the inability to distinguish the different incoming photons, which can render a time of flight ranging system inaccurate, since such a time of flight ranging system precisely measures the time between emission of a photon and receipt of the photon, after reflection from an object.
To that end, pulse shaping circuits are used to shorten the output pulses from a SPAD. An electronic device including such a pulse shaping circuit is now described with reference to FIG. 1. Here, the electronic device 100 includes a SPAD 102 coupled between a high voltage node VHV and a node N1 at the anode of the SPAD 102. A first transistor T1 is coupled between node N1 and ground and is switched by a signal Vquench, and a second transistor T2 is coupled between the node N1 and a pull up node VANA and is switched by an enable signal EN. An output inverter 104 has an input coupled to the node N1. The voltage at N1 is shown in FIG. 2, labeled as ANODE, and the output voltage output from inverter 104 is shown in FIG. 2, labeled as PIX_OUT. As can be seen, the pulse from PIX_OUT is substantial in duration, which as explained above is undesirable.
Therefore, the electronic device 100 includes a pulse shaper 106 that operates to truncate the PIX_OUT signal so as to produce a signal PS_OUT suitable for use. The pulse shaper 106 includes an inverter 108 receiving the signal PIX_OUT as input, and outputting the signal PS_INT, which is an in inverted version of PIX_OUT, as shown in FIG. 2. A delay block 110 is coupled to the output of the inverter 108 and generates a delayed version of PS_INT, shown as PS_DEL in FIG. 2. The delay block 110 is coupled to an inverting input of an AND gate 112, and the output of the inverter 108 is coupled to a non-inverting input of the AND gate 112. The resulting signal, PS_OUT, is a truncated version of PS_INT, and is suitable for use.
As can be seen from FIG. 1, the pulse shaping circuit 100 utilizes multiple logic components, and therefore consumes an undesirable amount of current and area. Since reductions in both power consumption and area are commercially desirable, new designs of pulse shaping circuits 100 are desired.